Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electric components. For example, a complex microprocessor chip may require many hundreds of connections to external devices.
Semiconductor chips have commonly been connected to electrical traces on mounting substrates using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on the microelectronic assembly, and fatigue of the connections due to changes in size of the chip and the substrate under thermal expansion and contraction.
Numerous attempts have been made to solve the foregoing problems. One structure that has been used to successfully address these problems is the "interposer" or "chip carrier", disclosed in commonly assigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,455,390. Interposers according to certain embodiments taught in these patents comprise a flexible, sheet-like element having a plurality of terminals disposed thereon. Flexible leads are used to connect the terminals with contacts on a microelectronic component such an integrated circuit. The terminals may then be used to test the microelectronic chip, and may be subsequently bonded to a final microelectronic assembly. The flexible leads permit thermal expansion of the various components without inducing stresses in the connection.
A compliant layer may be disposed between the microelectronic component and the flexible, sheet-like structure. The compliant layer encapsulates the leads and facilitates connection of the terminals to a test device and/or to the final electronic assembly by compensating for variations in component flatness and terminal heights.
Commonly assigned U.S. Pat. No. 5,518,964, hereby incorporated in its entirety herein, discloses further improvements in microelectronic connections. In certain embodiments of the '964 patent, a flexible, sheet-like element has a first surface with a plurality of elongated, flexible leads extending from a terminal end attached to the sheet-like element to a tip end offset from the terminal end in a preselected, first horizontal direction parallel to the sheet-like element. The tip ends have bond pads for connection to a microelectronic component. Each of the plurality of leads is simultaneously formed by moving all of the tip ends of the leads relative to the terminal ends thereof so as to bend the tip ends away from the sheet-like element. This is accomplished by relative movement between the sheet-like element and the microelectronic component.
The tip ends of the leads are initially attached to the sheet-like element. The initial position of the bond pads on the tip ends is thereby fixed in order to facilitate attachment to the microelectronic component.
During or after forming the leads by displacing the microelectronic element relative to the sheet-like element, a compliant material, such as silicone, is injected between the microelectronic element and the sheet-like element. The compliant layer facilitates testing by providing an even pressure on all the terminals located on the flexible sheet-like element regardless of the flatness of the testing fixture. Similar advantages are realized during final assembly.
In one method taught in the '964 patent for fabricating an assembly comprising an interposer and a microelectronic chip, a flexible, multilayer dielectric sheet-like element is stretched taut using mechanical means. While the multi-layer sheet is maintained in the taut condition, it is bonded to a ring-like generally circular frame so that the multi-layer sheet stretches across the central opening of the frame. The multi-layer sheet is bonded to the frame using a suitable hot temperature adhesive such as epoxy resin film, preferably on the order of about 10 microns thick. The frame is formed from molybdenum because that material has coefficient of thermal expansion substantially equal to that of the silicon semiconductor part with which the assemblage will be used in later steps. The flexible dielectric sheet-like element is maintained in its stretched, taut condition by the molybdenum ring until the end of the process. The sheet is therefore maintained in a stable, repeatable condition for formation of the leads and for bonding to the microelectronic component. The multi-layer dielectric sheet-like element is then processed in order to form the leads, terminals and bonding pads necessary to make connections between the microelectronic element and other components.
After terminals, leads, and other elements have been formed on the sheet-like element, the sheet-like element, together with the molybdenum ring, is placed in a fixture on top of and adjacent to a wafer containing an array of microelectronic chips. The fixture, part of a hot-air press or an autoclave, has ports for the pressurization and evacuation of volumes defined by the top and bottom surfaces of the wafer and the sheet-like element. The wafer and the sheet-like element are next aligned by bringing the sheet-like element into registration with the wafer. One or both of those components are moved in the horizontal x-y directions through the use of micrometer screw adjusting devices. A microscope or machine vision system may be used in conjunction with fiducial markings on the components in order to assist in alignment.
Because the sheet-like element is continuously held taut throughout the lead-forming process and the aligning process by the same molybdenum frame, the leads remain in a constant, stable position with respect to the sheet-like element and with respect to each other. Alignment with the wafer is therefore precise over its entire area.
While the wafer and the sheet-like element are maintained in precise alignment, compressed inert gas such as nitrogen is admitted in order to increase the pressure between a top plate of the fixture and the sheet-like element. This biases the sheet-like element downwardly towards the wafer so that a bonding material on the bond pads located at the tips of each lead is engaged with a contact on the microelectronic component. Because pressure from the compressed gas exerts force equally across the area of the sheet-like element, contact engagement across the area is assured regardless of the heights of the contacts.
While the gas pressure is maintained, the assembly is heated to a temperature sufficient to activate the bonding material at the bond pads in order to form metallurgical bonds between the tip ends of the leads and the contacts of the microelectronic components on the wafer. During this operation, the sheet-like component tends to expand at a rate greater than the rate of expansion of the silicon wafer. However, because the sheet-like element is held under tension by the molybdenum frame, the thermal expansion of the sheet-like element is substantially taken up in relieving the tensile stress. The actual movement of features on the sheet-like element due to thermal expansion is approximately equal to the thermal expansion of the frame. The frame, in turn, has a coefficient of thermal expansion substantially equal to that of the wafer. Therefore, features of the sheet-like element remain in alignment with features of the wafer during the heating process.
After the bond pads are bonded to the microelectronic component, an encapsulant is injected between the dielectric sheet and the wafer such a silicone. The encapsulant is a flowable, curable dielectric material such as silicone. The encapsulant may be injected between the dielectric sheet and the wafer immediately after bonding, whereby the force of the pressurized encapsulant acting on those components separates them and bends the leads, forming a compliant lead configuration.
Alternatively, the leads may be formed before injecting the encapsulant by retaining the wafer and the sheet-like element against moveable platens by vacuum, and moving the platens with respect to each other, bending and forming the leads. The encapsulant is then injected while the dielectric sheet and the wafer are in their displaced positions.
The injection operation is performed either using the same fixture in which the bonding step is performed or in a second fixture. The flowable material is constrained between the flexible sheet and the wafer, and does not cover the terminal features on the top surface of the dielectric sheet which remain exposed for testing and for permanent bonding to a microelectronic assembly.
After the flowable, curable dielectric material has been cured, the flexible sheet/microelectronic component assembly is removed from the fixture, trimmed and tested. The fixture is then reused to perform the above operations on the next flexible sheet/dielectric component assembly.
Still further improvement in the above-described process would be desirable.